The timeline lets you control when objects appear on a slide, and how long they appear for. You can also control the layer order that objects appear in.
Another way to change when an object appears on the slide is to align it to the playhead. The playhead is represented by a red box on the timeline, with a red line that descends through all of the objects below.
Click a spot on the timeline to move the playhead.
Select an object.
Click Modify on the menu bar.
Select Sync with Playhead.
The object moves on the timeline so that it will appear at the time marked by the playhead.